1. Field of the Invention
The present invention relates to an ink composition, an inkjet recording method using the same and a novel compound having a heterocyclic structure and a polymerizable group in the molecule. More specifically, the invention relates to an ink composition that is curable with high sensitivity upon exposure to active radiation and suitable for use in inkjet recording and can be cured into a product having sufficient flexibility even after curing, and also relates to an inkjet recording method and a novel polymerizable compound useful as a sensitizing dye.
2. Description of the Related Art
Examples of an image recording method for forming an image on a medium to be recorded such as paper on the basis of an image data signal include an electrophotographic mode, a sublimation type and fusion type thermal transfer mode and an inkjet mode. Since the electrophotographic mode needs a process for forming an electrostatic latent image on a photoreceptor drum by means of electrostatic charge and exposure, the system becomes complicated, resulting in high manufacturing costs. Also, in the thermal transfer mode, though an apparatus is inexpensive, an ink ribbon is used, and therefore, not only are the running costs high, but waste is generated.
On the other hand, in the inkjet mode, not only is an apparatus inexpensive, but an ink is discharged into only a required image part, thereby forming an image directly on a medium to be recorded. Therefore, the ink can be efficiently used, and the running costs are inexpensive. Furthermore, a noise is low, and this inkjet mode is excellent as an image recording mode.
As an ink composition which is curable upon irradiation with active radiation rays such as ultraviolet rays (radiation curable ink composition), for example, ink compositions which can be cured with high sensitivity to form an image with high image quality are demanded as an inkjet recording ink composition. By achieving high sensitization, advantages are achieved in that consumed electric power can be reduced and in that a load to an active radiation ray generator is decreased, whereby a high life span can be realized. Furthermore, by achieving high sensitization, since sufficient curing is achieved upon irradiation with active radiation rays, various advantages are achieved such as in that the vaporization of uncured low-molecular weight substances is suppressed and in that a lowering of the strength of a formed image is suppressed.
Compounds having polymerizing ability are useful for a variety of curable compositions. Compositions curable upon exposure to active radiation such as ultraviolet light are required to be curable with high sensitivity upon exposure to the radiation and to be capable of forming a high-quality cured film. Among such compositions, for example, colored curable compositions capable of forming a colored cured film are used as ink compositions and the like.
In recent years, a curable inkjet mode by ultraviolet light is receiving attention, because its odor level is relatively low and because it has quick-drying properties and allows recording on a non-ink-absorbing medium to be recorded.
Such a photopolymerizable or photo-curable ink composition uses a photopolymerization initiator, examples of which generally include benzil, benzoin, benzoin ethyl ether, Michler's ketone, anthraquinone, acridine, phenazine, benzophenone, and 2-ethylanthraquinone (see for example Japanese Patent Application Laid-Open (JP-A) No. 06-308727. Photopolymerizable compositions using these common photopolymerization initiators do not always have sufficient curing sensitivity and sometimes need a long time for exposure to light in image forming. When fine images are formed, such compositions have a problem in which if slight vibrations occur in the process of forming images, high-quality images cannot be formed. In addition, since the total amount of the energy radiation from the light source for the exposure can increase, a large amount of the associated heat radiation has to be taken into account.
It has been proposed that methods for increasing the sensitivity of radiation-curable polymerizable compounds to radiation should use a photopolymerization initiator in combination with a sensitizing dye, and the use of various polymerization initiating systems are disclosed. For example, it has been proposed that thioxanthone compounds should be used as sensitizing dyes (see for example JP-A Nos. 06-308727 and 56-143202). Even after the curing of the polymerizable compositions, however, such sensitizing dyes as thioxanthone compounds remain as monomers, which are not bonded to the other compounds in the cured film and therefore tend to cause a problem in which they can migrate to other materials in contact with the cured film, behave as low-molecular-weight components to reduce the strength of the cured film similarly to plasticizers, or cause the surface of the cured film to have stickiness so that the surface curing sensitivity can be reduced.
In the technical field using ink compositions, it is an important issue to prevent blocking between an image formed in an ink receiving layer and other materials in contact with the image, specifically, to prevent the transfer of ink from the image to the other materials or to prevent ink-induced undesirable adhesion to other materials. Therefore, it is proposed that polyfunctional thioxanthone compounds with good anti-blocking properties should be used for ink compositions (see for example Japanese National Phase Publication (Laid-Open) No. 2005-512973). However, the resulting ink compositions have high viscosity and therefore are not suitable for inkjet recording.